Operational amplifiers generally have complementary input terminals, a high input impedance, and a high gain, and they often have complementary output terminals. Such characteristics make operational amplifiers useful in a wide variety of applications. For example, operational amplifiers are frequently used in pipelined analog-to-digital (“A/D”) converters, such as the A/D converter 10 shown in FIG. 1. The A/D converter 10 includes a series combination of number of A/D stages 12 connected in series with each other. Each of the A/D stages 12 includes a respective sample-and-hold (“S/H”) circuit 14 having a sample output that is connected to the input of a processing stage 16. Each processing stage 16 generates a respective bit of a binary number corresponding to the amplitude of a signal applied to an input of the S/H circuit 14. As explained in greater detail below, each of the processing stages 16 includes an amplifier (not shown in FIG. 1) that alternates between sampling an input signal and amplifying it. In practice, the A/D stages 12 are driven by a clock signal (not shown) that periodically trigger the S/H circuit 14 in each stage 12. The clock signal also causes the amplifiers (not shown) in alternate stages 12 to sample while the amplifiers in the remaining stages amplify, and vice-versa. The bits generated by the respective processing stages 16 are applied to a digital error correction circuit 18 to generate a digital output signal indicative of the amplitude of an input signal VIN applied to the A/D converter 10. This digital output signal has the same number of bits as the number of bits from the processing stages 16.
As shown in FIG. 2, each of the processing stages 16 includes a single bit A/D converter 20, which generates a binary output. The output of the A/D converter 20 is also connected to the input of a digital-to-analog (“D/A”) converter 24, which generates a respective analog voltage. A subtraction circuit 28 subtracts the analog signal from the D/A converter 24 from the analog signal at the output of the S/H circuit 12. The resulting signal is amplified by an operational amplifier 30 to provide the input to the next S/H circuit 12.
In operation, each of the A/D stages 12 corresponds to a different amplitude level. The A/D converter 20 in the first A/D stage 12 generates a “1” bit at its output if the VIN signal has an amplitude that is greater than a relatively large threshold voltage. For example, for an A/D converter 10 having an operating range from 0-8 volts, the A/D converter 20 in the first A/D stage 12 may generate a “1” bit if the amplitude of the VIN signal is greater than 4 volts. If the A/D converter 20 generates a “1” bit, the D/A converter 24 in the same stage generates an analog voltage corresponding to the threshold, e.g., 4 volts. Using the above example, if the amplitude of the signal VIN is 5.25 volts, the A/D converter 20 will generate a “1” bit, and the D/A converter 24 will output 4 volts. The subtraction circuit 28 will then output a voltage of 1.25 volts (i.e., 5.25 volts−4 volts), which is passed on to the next A/D stage 12 after being amplified by the amplifier 30.
The second A/D stage 12 determines whether the received voltage is greater than a respective threshold that is less than the threshold of the first stage 12, such as half the threshold of the first stage 12. Thus, again using the above example, the second A/D stage 12 may determine if the amplitude of the received signal is greater than 2 volts. In a similar manner, the subsequent A/D stages 12 determine if the received voltage is less than an ever-decreasing threshold level. However, since the amplifier 30 amplifies the signal from the subtraction circuit 28 in each of the stages, the subsequent stages 12 need not process an ever-decreasing input voltage. For example, if amplifier 30 in the first A/D stage 12 has a gain of 2, the second stage 12 can effectively determine if the signal at the output of the subtraction circuit 28 in the first stage 12 is greater than 2 volts by determining if the output of the amplifier 30 in the first stage 12 is greater than 4 volts. Since the 1.25 volt output of the subtraction circuit 28 in the first stage 12 amplified by the amplifier 30 will be 2.5 volts, which is less than 4 volts, the A/D converter 20 in the second stage 12 will output a “0” bit, and the subtraction circuit 28 in the second stage 12 will not subtract any value from the 2.5 volt input. By determining if twice the 1.25 volt amplitude of the signal from the subtraction circuit 28 in the first stage 12 is greater than 4 volts, the second A/D stage 12 effectively determines if the 5.25 volt amplitude of the input signal VIN, less the 4 volt threshold value of the first stage 12 is greater than 2 volts. The amplifier 30 in the second stage 12 may also amplify the 2.5 volt output of the subtraction circuit 28 by 2 to output a voltage of 5 volts to the third A/D stage 12.
The third A/D stage 12 operates in the same manner as the first and second A/D stages 12 to compare the amplitude of the input signal to 4 volts. Since the received 5 volt signal is greater than 4 volts, the A/D converter 20 in the third A/D stage 12 outputs a “1” bit. However, in making this comparison, the third A/D stage 12 is effectively determining if the amplitude of the input signal VIN, less the threshold value of any stage 12 generating a “1” bit, is greater than 1 volt. The advantage of using processing stages 16 having an amplifier 30 is that the same circuit can be used for each of the processing stages 16, yet the downstream A/D stages 12 can process successively smaller voltage levels without any loss of resolution or accuracy.
As mentioned above, and as explained in greater detail below, the amplifier 30 receives a clock signal (not shown in FIG. 2) that alternates between sampling a signal applied to its input and then amplifying the sample.
The A/D converter 10 shown in FIGS. 1 and 2 can generate very precise indications of the magnitude of an analog signal by including a large number of A/D stages 12 since a large number of A/D stages 12 generate a correspondingly large number of bits. However, it can take considerable time for an input signal to propagate through all of the A/D stages 12 as the number of stages becomes larger. It is therefore important for the operational amplifiers 30 in the stages 12 to respond as quickly as possible to changes in voltage level. Unfortunately, the slew rates of common operational amplifier designs can be fairly slow, thus reducing the speed of A/D converters and other types of devices using such amplifiers.
An example of a typical operational amplifier 40 of conventional design is shown in FIG. 3. The operational amplifier 40 includes a first stage 42 implemented by the components in the center of the amplifier 40, and a second stage 44 implemented by the components on each side of the amplifier 40. The first stage 42 includes a pair of NMOS input transistors 50, 52 that receive respective input signals In+ and In− at their gates. The transistors 50, 52 are each coupled in series with respective sets of PMOS bias transistors 56, 58 and NMOS bias transistors 60, 62. Appropriate bias voltages are applied to the gates of these transistors 56, 58, 60, 62 so that they have a desired impedance and allow a desired level of current to flow between a supply voltage VCC and ground. The sources of the input transistors 50, 52 are coupled to each other so that they form a virtual ground with respect to the input signals In+ and In−. The outputs of the first stage 42 are a− and a+ at the respective junctions between the PMOS bias transistors 58 and the NMOS bias transistors 60.
In operation, the input transistors 50, 52 invert the respective signals In+ and In− applied to their gates so that a change in the voltage at node b− is inversely proportional to a change in the voltage of the input signal In+, and a change in the voltage at node b+ is inversely proportional to a change in the voltage of the input signal In−. For example, an increase in In+ and a corresponding decrease in In− cause an increase in current through the transistor 50 and a decrease in current through the transistor 52. The increased current flowing through the transistors 56, 58 on the left hand side of the first stage 42 causes the voltage at the a− output to decrease, and the decreased current flowing through the transistors 56, 58 on the right hand side of the first stage 42 causes the voltage at the a+ output to increase.
The outputs a− and a+are coupled to respective NMOS input transistors 70, 72 of the second stage 44. The transistors 70, 72 are each coupled in series with respective PMOS bias transistors 76 and respective NMOS bias transistors 78, which receive appropriate bias voltages at their gates to provide a desired impedance and allow a desired level of current to flow through the transistors 70, 72. Like the drains of the bias transistors 62, the drains of the bias transistors 72 are coupled to each other so that they form a virtual ground with respect to the input signals applied to the second stage 44.
In operation, a decrease in the a− signal responsive to an increase in In+ signal causes a decrease in the current flowing through the transistor 70. Similarly, an increase in the a+ signal responsive to a decrease in In− signal causes an increase in the current flowing through the transistor 72. The decreased current through the transistor 76 on the left hand side of the second stage 44 causes the voltage at the OUT+ output terminal to increase, and the increased current through the transistor 76 on the right hand side of the second stage 44 causes the voltage at the OUT− output terminal to decrease.
In practice, operational amplifiers like the amplifier 40 shown in FIG. 3 can be unstable and are therefore subject to oscillation. As a result, a compensation capacitor 80 is normally connected between the output terminal OUT+ and a node b−, and another compensation capacitor 82 is normally connected between the output terminal OUT− and a node b+. As explained above, the voltage at node b− is inversely proportional to In+, and the voltage at node b+ is inversely proportional to the input signal In−. Insofar as the voltage at the output terminal OUT+ is directly proportional to In+, the voltage at the output terminal OUT+ is inversely proportional to the voltage at node b−. Similarly, since the voltage at the output terminal OUT− is directly proportional to In−, the voltage at the output terminal OUT− is inversely proportional to the voltage at node b+. Therefore, the compensation capacitors 80, 82 provide negative feedback from the output of the second stage 44 to respective nodes b− and b+ of the first stage 42. This negative feedback stabilizes the amplifier 40 and keeps it from oscillating.
Although the use of the compensation capacitors 80, 82 has the desirable effect of stabilizing the amplifier 40, it also has the undesirable effect of reducing the slew rate of the amplifier 40. The compensation capacitors 80, 82 reduce the slew rate of the amplifier 40 because they provide very large negative feedback signals to the first stage 42 of the amplifier as the output terminals OUT+ and OUT− start to transition. As shown in FIG. 4, the OUT+ terminal starts to transition high at time t0 responsive to the input signal In+ transitioning high and the input signal In− transitioning low. However, a short time later, the very large negative feedback signals coupled through the compensation capacitors 80, 82 actually cause the transition of the OUT+ signal to reverse direction and transition negatively until time t1. Thereafter, as the capacitors 80, 82 become charged, the transition of the OUT+ resumes its positive direction. However, the OUT+ signal does not reach the high logic level until time t2, which is substantially after it would reach that level but for the period of negative transition prior to time t1. When the amplifier 40 is used in a pipelined A/D converter, such as the A/D converter 10, the reduced slew rate caused by the compensation capacitors 80, 82 can significantly increase the time required for the A/D converter to provide an output indicative of the magnitude of an analog signal.
There is therefore a need for an amplifier that has an enhanced slew rate so that it can be advantageously used in a variety of applications, including being used in pipelined A/D converters.